1. Field of the Invention
This invention is directed to metallic devices inserted in the irregular space between the periphery of the array of rectangular fuel assemblies forming the core of a nuclear reactor and the cylindrical core barrel surrounding the core for reflecting neutrons back to the peripheral fuel assemblies to thereby raise their efficiency. More particularly, it is directed to neutron reflectors which fit between the former plates commonly found in this irregular space in existing reactors and are secured to the core barrel by a mounting which accommodates for thermal expansion of the metallic mass in all axes.
2. Prior Art
In a commercial nuclear reactor, the fissionable material is contained in square, elongated fuel assemblies which are massed to form a generally cylindrical core. The core is housed inside a cylindrical, stainless steel core barrel between horizontal upper and lower stainless steel core plates. This entire assembly is mounted inside a cylindrical pressure vessel with hemispherical upper and lower heads. Reactor coolant, introduced into the pressure vessel through inlet nozzles, flows downward in the annular space between the core barrel and pressure vessel, reverses direction, flows upward through openings in the lower core plate, and through the fuel assemblies where it is heated as a result of fission reactions before being directed radially out of the pressure vessel through outlet nozzles.
Since the fuel assemblies are square in cross-section, an irregular space exists between the periphery of the core and the inner surface of the core barrel. The usual practice is to place longitudinally extending flat plates along the outer surfaces of the fuel assemblies to confine the upward coolant flow to the fuel assemblies. These plates are held in place by horizontal, irregularity shaped, former plates bolted to the longitudinal plates and to the core barrel. Holes in the former plates permit limited coolant flow in the generally annular space between the longitudinal plates and the core barrel to provide cooling for these components and to equalize the pressure on both sides of the longitudinal plates.
Although the original purpose of the vertical plates was to channel reactor coolant flow through the fuel assemblies, it has been recognized that to some extent, they also reflect neutrons back toward the peripheral fuel assemblies. However, since these plates are relatively thin, most of the neutrons escaping radially from the core migrate into the large volume of water between the plates which absorbs or thermalizes the neutrons with very little reflection.
It has subsequently been recognized in commonly owned U.S. patent application Ser. No. 576,655 filed on Feb. 3, 1984, that replacement of the water in the space between the core and the core barrel with essentially non-hydrogen containing materials results in much more efficient radial neutron reflection. Specifically, it is suggested that this space can be filled with vertically stacked, generally annular, stainless steel plates cooled by reactor coolant circulated through a few widely spaced vertical bores or a number of elongated metal cans arranged around the periphery of the core. These cans may contain either blocks of zirconia or stainless steel, again, with vertical cooling ports, or a number or rods supported at several levels by horizontal plates. These rods are either solid stainless steel or zirconia discs encassed in zircalloy. In all cases, the weight of the reflector units is supported by the lower core plate and the assemblies are held in place by securing them to the upper and lower core plates.
Commonly owned U.S. patent application Ser. No. 670,732 filed on Nov. 13, 1984, discloses removable, modular neutron reflectors each comprising a metallic box in which an array of closely packed steel rods is suspended. The interstices between the closely packed rods form passages for reactor coolant to circulate and comprise about 8-10% of the volume so that 90-92% of each module is metallic material. Each module is suspended in the irregular space between the fuel assemblies and the core barrel by a single point mounting on the core barrel which allows for free radial, circumferential and longitudinal thermal expansion of the module. The single point mounting comprises four closely spaced bolts torqued to clamp the module near its upper end to the core barrel. A large pin at the center of the pattern of bolts fixes the position of the module and serves as a redundant support. Each of the modules extends along the full height of the reactor core with lateral displacement of the lower end being restrained by a downwardly extending pin which slides in a vertical bore in the lower core plate.
While these reflector units are effective and have their own advantages, they all extend the full length of the reactor core and thus, require a major modification to the reactor internal design. There is a need for neutron reflector units which substantially fill the irregular space between the fuel assemblies and the core barrel with neutron reflecting material without requiring removal of the former plates or other modifications to the internals.